The problem of powering such portable or similar receivers as by expendable power sources, such as batteries and the like, has received considerable attention in the art over relatively recent years, particularly in connection with portable cellular telephones and pagers and the like, and a large number of patents have been issued to try to address power or battery conservation to ensure long functioning of the receiver. Typical of such approaches in the field of cellular telephones and mobile communication systems, are disclosed, for example, in U.S. Pat. Nos. 4,860,005; 4,961,073; 4,987,317; 5,023,932; 5,031,231; 5,054,052; 5,5,089,813; 5,095,308; 5,128,938; 5,140,698; 5,150,364; 5,175,870; 5,230,084; 5,237,603; 5,293,639; 5,301,225; 5,376,975; 5,384,564; 5,392,287; 5, 392,457; 5,392,462; 5,404,578; 5,406,613; 5,428,638; 5,440,229; 5,440,650; 5,448,756; 5,465,392; 5,465,394; 5,471,655; 5,475,374; 5,483,672; 5,487,181; 5,507,039; 5,507,040; 5,517,679; 5,519,762; 5,530,911; 5,541,976; 5,541,929; 5,542,116; 5,542,117; 5,561,693; 5,568,513; 5,570,025; 5,590,396; 5,594,951; 5,606,313; 5,606,728; 5,606,739; 5,613,235; 5,627,833; 5,628,054; and 5,640,441. Typical proposals adapted for such problems in radio pagers are exemplified by, for example, U.S. Pat. Nos. 4,652,875; 4,691,382; 4,745,408; 4,755,816; 4,779,091; 4,940,975; 5,001,471; 5,025,251; 5,150,954; 5,193,211-2; 5,274,843; 5,420,576; 5,459,457; 5,508,688; 5,556,081; 5,625,884; and 5,649,314. Other communication and more general power saving or controlling circuits are exemplified in U.S. Pat. Nos. 4,821,309; 4,893,094; 4,903,335; 4,905,271; 4,977,611; 4,996,526; 5,027,428; 5,101,510; 5,103,192; 5,109,530; 5,111,201; 5,204,986; 5,222,245; 5,421,691; 5,251,325; 5,265,270; 5,278,521; 5,299,117; 5,327,172; 5,357,245; 5,359,594; 5,361,397; 5,389,930; 5,422,681; 5,423,057; 5,423,077; 5,430,441; 5,438,696; 5,438,701; 5,440,556; 5,463,382; 5,465,400; 5,513,385; 5,524,021; 5,525,992; 5,533,058; and 5,592,171.
As will hereinafter more fully be explained, the present invention primarily relates to a transmitter-receiver environment very different from cellular and paging communication systems, being concerned, rather, with vehicle-recovery systems of the type described in U.S. Pat. Nos. 4,818,998 and 4,908,629, and currently in operation under the trademark "LoJack", (described, also, in the assignee's 1989 brochure entitled "LoJack Stolen Vehicle Police Recovery Network"), where a vehicle-carried transponder receiver can only receive an activation signal from a transmitter or transmitters in its area intended for that receiver when the vehicle has been reported missing and the transmitter or transmitters is or are so alerted to transmit such an activation signal. In current vehicle-recovery systems of this character, the vehicle receiver is continually powered, as from the car battery; but the present invention looks to miniaturized receivers that can be well hidden in the vehicles and that are not dependent upon the vehicle battery. The receiver, rather, is to have self-contained limited-life small consumable batteries, the power drawn from which is thus to be conserved and budgeted for use only accordingly to wake-up a normally dormant receiver to look for a possible activation transmission specifically intended for it, and to stay awake (with transponder response where appropriate) only if its stolen or missing vehicle activation signal is being transmitted--otherwise, to return to dormant condition.
This is therefore a very different environment embracing very different problems underlying power conservation than those involved in the above-mentioned cellular phone or paging environments, such that energy-conserving techniques proposed for the latter, including those described in the above groups of patents, are inapplicable and insufficient for the purposes of the present invention. The different environment of such vehicle-recovery systems, indeed, unlike the synchronized paging transmitters sending the same message at the same time, or the two-way cell system control of cellular phones, resides in the vehicle-recovery system use of a series of transmitters over a geographical area, say eight transmitters per control center, each of which (when a vehicle is reported stolen) transmits sequentially, in a different time slot, with the time slots being controlled accurately in time either from a time standard received from WWV (the national standards bureau in Boulder, Colo.), or from the time standard of global position satellites (GPS).
Since paging receivers are most often to be worn on the person, small size and weight are major marketing considerations, and in turn make reduced battery size and longer battery operating life of great concern. Paging, however, operates primarily one way, although there are some two-way systems, and such differ from the vehicle recovery system of the invention in two major functions. First, in a paging network, which consists of many transmitters, there is essentially a seamless network of transmitters in its coverage area, and the transmitters are synchronized so that all the transmitters transmit the same message at the same time. The transmission system does not need to know where the receiver is located so long as the paging receiver is located somewhere in its coverage area so that it will receive the message. This opens up some significant power-saving opportunities and advantages in the paging world. They can set timing; they can have the receiver turn itself off and come back on at a specific time; and also they can set that time so that the receiver knows exactly when its message is going to be sent. The receiver, moreover, does not have to know where it is in order to know what time to listen.
In the vehicle recovery system, on the other hand, the same message is not sent at the same time over the transmitter network geography; and, since the primary mode of use is in a vehicle, there is every reason to suspect the vehicle will be moving from one area to another. Since, as before pointed out, the vehicle-recovery transmitters are operated in a sequential system, further as opposed to a synchronized paging system, the receiver has no way of knowing where it is when it listens for a signal and requires highly novel algorithmic assistance, which is entirely foreign to paging receivers and their operation. Another advantage paging receivers have over vehicle recovery systems, of course, is that there are many more of the synchronized transmitters around, so the paging signals are many times greater than those of the signal environment of vehicle-recovery systems.
Cellular phones take a somewhat different approach in phone energy conversation systems, moving from area to area, but having the ability to communicate with the cell station which knows the parameters and can tell the cell phone to listen. Since the cell system controls the cell phone and is passing it from cell to cell, it can hand off the appropriate parameters as to just when to listen. The vehicle-recovery system of concern with the present invention, as earlier noted, does not have a two-way system and has no way of breaking into the network and requesting the information needed. The problems of conserving receiver battery power are thus much harder to deal with because the receiver does not know where it is at any given time and it can never be sure of exactly when to listen. When it does get out of phase or synchronization, moreover, it does not have the ability of the cellular phone systems to send a message back to the controlling environment and request information on what to do.
The vehicle receiver, when the vehicle is stolen, can be anywhere; in fact, in the normal course of such events, it will be moving from one area to another area. While this is not a problem if the receiver is turned on all of the time, such operation consumes the battery continuously and cannot solve the conservation problem underlying the present invention; namely, fundamentally, turning the receiver off when there is no existing transmitted signal for it, and with minimal turn-on or wake-up for monitoring for such a signal. When the receiver is off, indeed, it also has to know at what time to turn back on to look for a next message signal intended for it. If it has moved from one geographical area to another geographical area, then the time at which the receiver must activate itself has now changed; and all this without knowledge as to what that new time is or the location of the receiver.
The present invention has admirably solved these problems, providing a highly novel technique of (1) knowing when to wake up the receiver so as not unduly to use available power listening when there is no information actually transmitted, and (2), when the receiver is awakened and activated, consuming power in a highly intelligent manner that rapidly determines if the signal messages are not intended for that receiver, and accordingly turning off, so as not unduly to waste power decoding messages not intended for the receiver.